1. Field of the Invention
The present invention relates to an electric traction propulsion system for a vehicle, in particular a non-rail vehicle.
2. Background Information
The publication "VDI-Berichte No. 878, 1991," Pages 611 to 622, for example, describes electrically-powered, non-rail vehicles, whereby the wheels are propelled by electric motors. On such vehicles, one or more electric motors are supplied with electric operating power from an on-board power system, and the torque generated in the electric motors is transmitted to the drive wheels of the vehicle. Electric motors which can be used here are rotating field or synchro or induction motors of any desired construction, with a rotating field or induction winding which can be divided into a plurality of phase windings. Permanent magnet external rotor motors have been found to be particularly well-suited for this application. The phase windings of the electric motors are powered by a plurality of converters which correspond to the individual motors with phase-shifted, pulsing driver currents. These converters are electronic converters which control the pulsation rate and the amplitude of the driver currents by means of electronic semiconductor valves as required by a traction control device on the vehicle, e.g., by means of an accelerator pedal or a similar device. The converters, in turn, are fed from an on-board power supply system which can be driven by means of a generator, which is driven in turn by an internal combustion engine, for example, but which can also be powered by a rechargeable battery. Differential transmissions and step-down or reducing transmissions can also be used, if necessary, to transmit the torque which is generated by the electric motor to one or more drive wheels.
An electric motor used for a vehicle propulsion system must usually produce a relatively high level of power over a broad range of speeds. The phase windings are thereby fed by the converters with driver currents of up to 1000 .ANG. at up to 2 kHz. At a given frequency and intensity of the driver current, it is also possible, by changing the circuit configuration of the rotating field windings, to change the torque and the speed of the electric motor. As disclosed in EP-A-340,686, for this purpose, the number of phase windings connected in series can be changed by means of a plurality of controllable switches. For this purpose, mechanical switches are generally used, the moving contact elements of which can be adjusted, for example, by means of an electric actuator. On vehicles of the type described above, it is also conventional, in the on-board power supply or in the current leads to the phase windings, to incorporate switches which can de-energize the transmission of power to the electric motors, e.g., in the event of an emergency situation.
The very high accelerations which occur under some circumstances during operation of the vehicle, e.g., when it travels over bumpy ground, can result in significant mechanical loads and thus significant premature fatigue of the structural components of the vehicle, and also of the components of the drive train which transmit the torque. Mechanical switches in the electrical operating power path of the electric motors are particularly susceptible to damage as a result of the acceleration which occurs during travel. An acceleration applied to such a switch can open a switch contact which is normally closed, if the inertial force of the contact element and/or of the actuator connected to this contact element which is applied to the moving contact element on account of the acceleration, overcomes the force of a spring which protects the closing of the contact element. If the switch is carrying current at the time this acceleration occurs, a spark occurs which, at the current intensities indicated above, can result in a significant erosion of the contact element, or even in a destruction of the switch. The measures adopted to counteract this situation generally include making the switches heavier-duty by using correspondingly stronger contact springs, but that in turn exerts an increased force on the actuators which actuate the switches, and thus increases the weight of the entire switching system.